Improvements in semiconductor field-effect transistors (FETs) have traditionally been implemented by scaling down the relative dimensions of the device. However, because of fundamental scaling limits, advanced field-effect transistors and the complementary metal oxide semiconductor (CMOS) circuits in which they can be found are increasingly relying on nontraditional materials and special substrate geometries to achieve desired improvements in circuit performance. High-mobility channel field-effect transistors in which the channel material comprises a high-mobility material such as germanium instead of the traditional silicon are one example of a type of field-effect transistor incorporating nontraditional materials. Germanium-channel devices are expected to provide significant performance advantages since electron and hole mobilities are two to four times higher in germanium than in silicon. Typically the germanium layer that would become the channel would be epitaxially grown on an underlying layer of silicon or a silicon-containing semiconductor.
There are many challenges in growing epitaxial layers of germanium on silicon. Germanium layers grown directly on silicon must be thin because the critical thickness for films with the 4% lattice mismatch of silicon and germanium is only a few nanometers. Films exceeding this critical thickness spontaneously form dislocations to relieve the strain, consequently degrading their value as device layers.
Epitaxial germanium films grown directly on silicon are typically formed by techniques such as rapid thermal chemical vapor deposition (RTCVD) and ultra-high-vacuum CVD (UHVCVD). A problem with these techniques is that the deposition rates at the early stages of film growth are highly variable, depending sensitively on the cleanliness of the silicon growth surface and the degree to which the growth surface is covered by germanium adsorbed from the process gas mixture. It is therefore nearly impossible to reliably grow a thin germanium film to a specified thickness. An additional problem is that thin germanium films tend to have an island structure at early stages of growth, which leaves them rough and discontinuous right in the thickness range of interest for device applications. While thick films could be grown and then thinned, thick films are invariably rough and contain defects that propagate back to the bottom-most portion of the film.
In principle, thin and smooth layers of germanium may be formed on silicon by deposition methods such as sputter deposition, evaporation, and plasma-enhanced chemical vapor deposition (PECVD), and then crystallized using the silicon substrate as a template. However, the silicon/germanium interface is usually too contaminated to provide a good surface for recrystallization and these methods are inherently spatially non-selective (e.g., same deposition rate on silicon and silicon oxide).
There have been some limited suggestions of electrodeposition of germanium on metals. However, the attempts have not been especially successful. This may be due to the high instability of the germanium chemistry and, at the same time the very low hydrogen overpotential on germanium surfaces. Therefore, all the plating current is used for the proton reduction (side reaction) and no germanium electroplating once a limited amount of Ge is formed on the electrode surface has been reported.
For instance, plating of germanium on metal substances has been reported in alkaline aqueous solution [see Fink et al., Journal of the Electrochemical Society, vol. 95 (1948)] and in glycol solutions [see Szekely, Journal of the Electrochemical Society, vol. 98, p. 318 (1951) and U.S. Pat. No. 2,690,422 to Szekely]. More recently, some studies have been reported which were directed to germanium nucleation studies from ionic liquid media, for example, butyl-methyl-immidazolium hexafluorophosphate. [see Endres, Electrochemical and Solid State Letters, vol. 5, p. C38 (2002); Endres et al., Physical Chemistry and Chemical Physics, vol. 4, p. 1640 (2002); and Endres Physical Chemistry and Chemical Physics, vol. 4, p. 1649 (2002)].
In the aqueous approach, an extremely alkaline solution (pH>13) was used to minimize the proton concentration and therefore suppress the reduction of proton. This approach is not compatible with most microelectronics processes due to the extremely high pH of the electrolyte, which causes damage to most of the structures that are built from silicon oxide and other dielectrics and used in the fabrication of electronic devices.
In the non-aqueous solution approaches, where pure glycol or ionic liquid solvents are used, the free proton concentration in the solution is substantially reduced, eliminating the proton reduction and other side reactions. However, techniques using ionic liquids suffer from the disadvantages of high viscosity and high cost of the solvent materials.
Electroplating of germanium on semiconductor substrates such as silicon has not been previously achieved, in part, as mentioned above, because of the susceptibility of silicon to corrosion in high pH alkaline solutions.
Also, electroplating on semiconductor substrates can be more challenging than electroplating on metallic substrates, since semiconductor substrates have a substantial electron energy band gap. Depending on the semiconductor substrate used and the chemical species in the electrolyte, certain electrochemical reactions are not even possible. In addition, even if the germanium plating on semiconductor substrates is possible, there is no reason to believe that it would ever be possible to achieve high quality epitaxial germanium by plating. The electrodeposition temperature is too low to grow crystalline germanium directly, and annealing amorphous germanium electrodeposited on silicon to induce solid phase epitaxy (SPE) templated from the silicon substrate requires an ultraclean interface between the silicon substrate and the germanium electrodeposit, a seemingly unlikely prospect in a liquid plating solution environment.